Dr Ellen Clarke and PhD student Emily Herring promised to transport us to a tropical paradise in this fourteenth instalment of the HPS in 20 Objects Lectures, and they did not disappoint. The object of April’s lecture was coral, a small marine invertebrate closely related to the sea anemone and jellyfish. Corals, Ellen and Emily told us, are, and have been, the source of an enigma: corals disrupt the human tendency to divide life into discrete categories.

Animal, vegetable, or mineral? It seems easy to know to which of these options any particular lifeform belongs, but as Emily quickly pointed out, which of the boxes corals belong in have been historically tricky. In fact, before the 18th century, most naturalists used to believe that corals must be either a mineral or a plant! The ancient Greek philosopher Aristotle even had a word, ‘zoophytes’, for organisms that fell somewhere between the categories of “plant” and “animal”, and it’s likely that he included corals in this category.

So confounding are corals, that an explanation for it found its way to into Roman poetry where Ovid offers a mythological account of its existence. Most of us know about the great hero, Perseus, who, in the Metamorphoses, Ovid describes as killing the Gorgon, Medusa. Medusa, it’s told, had the ability to turn those that gaze upon her into stone. On his journey back home with Medusa’s, Perseus slays a sea monster with the aid of his new trophy. After, he washes his hands in the salt water, placing Medusa’s head on some underwater plants such that it would not be bruised by the sand. The powers of the head, Ovid tells us, affected the plants such that they turned to stone. Water nymphs see this, and after dispersing the seeds of the plants, fill the oceans with corals—plants that are soft below the waves but turn to stone above the water’s surface.

Not everyone believed corals to be plants or minerals, however. Al-Biruni (973-1048), an Iranian scholar hypothesised that corals were neither a plant nor a mineral, but rather an animal because they responded to touch. This idea didn’t really take off until the eighteenth century, however, until a French physician by the name of Jean-André Peyssonnel (1694-1759) tested corals in a variety of different ways. Doing this, he noticed that they responded to the touch. This observation led Peyssonnel to believe corals were animals. By the 1770s it was believed that the coral reefs were caused by the growth of the animals.

Henri Bergson (1859-1941) would later ask what the primary difference between animals and plants were. He believed that the overlap of animal and plant properties that existed in organisms such as corals blurred the boundaries between the two kingdoms. When asked which characteristics animal life had that plant life did not, and vice-versa, Bergson concluded that no such characteristic existed; plants and animals then must not be static entities, according to Bergson, but points on a spectrum.

Around the same time Bergson was working, the zoologist Julian Huxley (1887-1975), started forming his own metaphysics inspired on Bergson’s writing. Huxley began thinking about what it meant to be an individual. Regarding the matter, he concluded that individuality was not a static category, but was rather like everything else in the living world in that it was something that had evolved and continued to evolve. Individuality came in degrees, according to Huxley, such that an individual coral polyp could be considered an individual, but so couldn’t a coral colony.

The lecture then turned from history to philosophy as Emily handed over to Ellen. Ellen then explained a contemporary problem in biology and philosophy of biology that corals stand as an example of. Huxley’s question, “where is the individual now?” is still pertinent. How do biologist and philosophers of biology know how to distinguish a single living thing from many? This problem isn’t as difficult for many of the organisms that we interact with on an everyday basis, but some organisms are not as easy to distinguish the individual from the many. Corals, as you might have guessed given the lecture’s topic, are such a problematic case.

Many corals are colonial, meaning that at some point in their life, the coral polyp will begin to “bud”, making a colonial copy of itself. When doing this, a polyp will either rip itself in two with each half regenerating into a full polyp, or cells will begin to grow outside of the polyp making a larger object. Understanding this phenomenon, the scientist and philosopher are confronted with a problem: what is the individual? Is a polyp a separate organism? Or is the polyp only a small part of a larger organism? The answer is unclear.

Coloniality isn’t the only puzzle one faces when studying corals. Colder water is better at sustaining life than warm water. To help survive the warm water in which they live, some corals make friends in the form of symbiotic relationships with a species of algae that lives inside its tissue. The algae give the coral nutrients and oxygen, and the coral gives the algae protection and carbon dioxide and other required nutrients.

Because of this and other examples of symbiotic relationships, some have argued that a host and all of its symbionts count as a larger organism—a “holobiont” organism. However, this position has not persuaded everyone. Instead of understanding the individual as the larger “holobiont” organism, some biologists and philosophers have suggested that it is more accurate to view the host and its symbionts as separate organisms. Once again the answer is unclear, and the conversation regarding it continues.

Coloniality and symbiosis illustrate how difficult it is to determine what counts as an individual coral. These cases and the puzzles that they reveal are still discussed lively today as much (if not more) than they were back when Bergson and Huxley investigated the issue. What Ellen and Emily’s presentation showed us, is that individuality comes in degrees and that there are no neat, tidy boxes in biology.

The lecture ended with Ellen speaking about the current disappearing coral reefs brought upon by threats such as invasive predators, pollution, and rising sea temperatures. In times of the stress brought upon by these and similar threats, the corals eject their symbionts, causing them to lose their colour and “bleach”, which can cause them to die if they don’t quickly regain their symbionts. Although these events are known to have happened before, the amount of bleaching that has recently occurred leads experts to predict that most of the world’s coral reefs will be destroyed by 2050. Despite this bleak revelation, a ray of hope is illuminated: humans are attempting to save the reefs, such as marine parks, coral reef nurseries, “designer reefs”, and coral probiotics. Ellen also showed us that reefs have disappeared before, but given their resilience have come back. This led her to conclude that the reefs may have a better chance of recovering from the destruction we are causing on this planet than we have—corals may be back way after we are gone.

The whiteness of the reef means that it has been bleached. (Photo credited to Laura Sellers, follow on Twitter at @LauraSellers11).